1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus, which includes a collector that collects particles in exhaust gas (exhaust particles) of an internal combustion engine.
2. Description of Related Art
Recently, in a diesel internal combustion engine mounted on a vehicle, a collector that collects exhaust particles is provided in a middle of an exhaust pipe in order to reduce the exhaust particles in exhaust gas.
The collector is generally a ceramic porous media that includes multiple exhaust gas passages. The collector adsorbs and collects the exhaust particles when the exhaust gas passes through porous partitions that define the exhaust gas passages. When the collected exhaust particles, which are collected by the collector, remains in the collector, a pressure loss is increased so that an efficiency of the engine may be degraded. Thus, the collector needs to be regenerated by burning (oxidizing) the exhaust particles in the collector when a computed value of a collected amount reaches a predetermined value.
Specifically, the collector carries an oxidation catalytic converter. At a time of the regeneration of the collector, after a main inaction, a post injection is made to supply a hydrocarbon (HC) into the collector such that an interior of the collector is heated due to a catalytic reaction of the HC and the exhaust particles collected in the collector are burned and removed.
Estimation method for estimating a collected amount of the exhaust particles in the collector includes a pressure-difference-based estimation method and an operational-state-based (operational-history-based) estimation method. In the pressure-difference-based estimation method, the collected amount is estimated based on a pressure difference across the collector. In the operational-state-based estimation method, the collected amount is estimated by integrating an increase of the collected amount. Here, the increase is computed based on a discharge amount of the exhaust particles from the internal combustion engine. Further, there is disclosed another method, in which the pressure-difference-based estimation method and the operational-state-based estimation method are combined. In this method, a correction coefficient is computed based on the increase of the collected amount computed through the pressure-difference-based estimation method and based on the increase computed through the operational-state-based estimation method. Then, an estimated value of the collected amount, which is estimated through the operational-state-based estimation method, is corrected based on the correction coefficient (see e.g., Japanese Unexamined Patent Publication No. 2004-76589, corresponding to U.S. Pat. No. 6,907,873).
In the above pressure-difference-based estimation method, the collected amount can be highly accurately estimated in a state where a flow rate of the exhaust gas is substantially high and an operational state of the internal combustion engine is reliably stable. However, in a transient operational state, such as an acceleration or deceleration state, where the pressure difference and the flow rate of the exhaust gas are transiently changing, the collected amount cannot be substantially accurately estimated. Also, when the flow rate of the exhaust gas is low, the pressure difference becomes small, resulting in that the collected amount cannot be substantially accurately estimated.
In a case where the degree of accuracy in estimation by the pressure-difference-based estimation method needs to be exclusively improved, the pressure-difference-based estimation method can be applied only when the operational state is substantially stable and the flow rate of the exhaust gas is substantially high. However, in this case, the pressure-difference-based estimation method is used only in a limited opportunity. Thus, the pressure-difference-based estimation method, which can highly accurately estimates the collected amount, is less often used to estimate. Therefore, even in a case of a semi-steady operational state, where the flow rate of the exhaust gas and the operational state are changing to some extent, the collected amount needs to be estimated by the pressure-difference-based estimation method in order to frequently use the pressure-difference-based estimation method for estimation.
In the semi-steady operational state, the degree of accuracy in estimation by the pressure-difference-based estimation method may be degraded. However, in an apparatus disclosed in Japanese Unexamined Patent Publication No. 2004-76589, the collected amount computed by the pressure-difference-based estimation method is not corrected. Therefore, under a state (e.g., the semi-steady operational state) where the degree of accuracy in estimation by the pressure-difference-based estimation method is degraded proper timing for the regeneration of the collector may be missed in a worst case scenario, and this may lead to a failure of the collector so that a function may be severely damaged.
The collected amount is computed as follows in a known apparatus for estimating the collected amount by switching the pressure-difference-based estimation method and the operational-state-based estimation method in accordance with the operational state. When the operational state is switched from the pressure-difference-based estimation method to the operational-state-based estimation method, the increase of the collected amount, which is estimated based on the operational state of the internal combustion engine, is added to a base estimated value, which is a current estimated value of the collected amount estimated through the pressure-difference-based estimation method. In this case, if the base estimated value of the collected amount is computed during the semi-steady operational state, the degree of the accuracy in estimating the collected amount by the operational-state-based estimation method may be degraded because the degree of the accuracy of the base estimated value of the collected amount itself is low.